git-gui: Show special diffs for complex conflict cases.

git-gui: Show special diffs for complex conflict cases.

Add special handling for displaying diffs of modified/deleted,
and symlink/mode conflicts. Currently the display is completely
unusable for deciding how to resolve the conflict.

New display modes:

1) Deleted/Modified conflict: e.g.
	LOCAL: deleted
	REMOTE:
	[diff :1:$path :3:$path]

2) Conflict involving symlinks:
	LOCAL:
	[diff :1:$path :2:$path]
	REMOTE:
	[diff :1:$path :3:$path]

In order to be able to display multiple diffs, this
patch adds a queue of commands to call.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>